Recently, the apparatus for running a car at constant speed, which automatically controls the car to run at the set speed, has been provided for the car.
In the conventional apparatus for running a car at constant speed, when a speed of the car reaches a desired speed during driving, this speed of the car is set by carrying out a setting operation by the driver; the running speed after setting is compared with the set speed of the car; a throttle valve is closed at the time when the running speed is larger than the set speed; and the throttle valve is opened at the time when the running speed is smaller than the set speed, thereby controlling the running speed near the set speed. Further, in the conventional apparatus for running a car at constant speed, when the driver operates a braking pedal and a clutch under a state such that the driver drives the car to run at a desired constant speed, a microcomputer in the apparatus for running a car at constant speed receives signals from a canceling switch which is actuated by the operation of the braking pedal and the clutch, and a running state of constant speed is canceled by a signal outputted from this microcomputer.
However, in the conventional apparatus for running a car at constant speed, owing to the surface of the road and the force of the wind, a deviation between the running speed and the set speed becomes large, or acceleration and deceleration are repeated around the set speed, and therefore response for making the actual running speed close to the set speed becomes slow, and there is such a problem as to give undesirable feelings to the driver.
Besides, in the conventional apparatus for running a car at constant speed, when the microcomputer is partially damaged by static electricity or electromagnetic noise etc., thereby resulting in a state such that the microcomputer does not operate to follow the program; the microcomputer does not accept the signal from the cancel switch, and the running state of constant speed can not be canceled. Furthermore, when the microcomputer runs away from the program in a manner to increase the speed of the car, it may become impossible to safely drive the car, and there exists such a problem as to fall into a very dangerous state.